OSPF Routing Protocol: Popular and Robust

Traffic Cop For Your Routing Domain

October 22, 2008

Open Shortest Path First is a robust link-state interior gateway protocol (IGP). People use OSPF when they discover that RIP just isn�t going to work for their larger network, or when they need very fast convergence. This installment of Networking 101 will provide a conceptual overview of OSPF, and the second part of our OSPF coverage will delve a bit deeper into the protocol itself, as well as OSPF area configurations.

OSPF is the most widely used IGP. When we discuss IGPs, we�re talking about one routing domain, or Autonomous System (AS). Imagine a medium-sized company with multiple buildings and departments, all connected together and sharing two redundant Internet links. All of the buildings on-site are part of the same AS. But with OSPF we also have the concept of an Area, which allows further segmentation, perhaps by department in each building.

To understand the design needs for areas in OSPF, let�s start by discussing how OSPF works. There�s some terminology you may not have encountered before, including:

Router ID: In OSPF this is a unique 32-bit number assigned to each router. This is chosen as the highest IP address on a router, and can be set large by configuring an address on a loopback interface of the chosen router.

Neighbor Routers: two routers with a common link that can talk to each other.

Adjacency: a two-way relationship between two neighbor routers. Neighbors don�t always form adjacencies.

LSA: Link State Advertisements are flooded; they describe routes within a given link.

Hello Protocol: this is how routers on a network determine their neighbors and form LSAs.

Area: a hierarchy. A set of routers that exchange LSAs, with others in the same area. Areas limit LSAs and encourage aggregate routes.

OSPF is a link-state routing protocol, as we�ve said. Think of this as a distributed map of the network. To get this information distributed, OSPF does three things.

First, when a router running OSPF comes up it will send hello packets to discover its neighbors and elect a designated router. The hello packet includes link-state information, as well as a list of neighbors. Providing information about your neighbor to that neighbor serves as an ACK, and proves that communication is bi-directional. OSPF is smart about the layer 2 topology: if you�re on a point-to-point link, it knows that this is enough, and the link is considered �up.� If you�re on a broadcast link, the router must wait for an election before deciding if the link is operational.